Gardening Articles: Edibles :: Fruit & Nut Trees

Tomato Blight: Breeding for Resistance

by Pat Stone

Late blight on tomato leaves

It's got to be an axiom of home gardening that nothing ruins a good garden like a vacation. Go away, come back and something has gone wrong. Still, even I didn't expect last summer's disaster. When my family returned from our annual week at the beach, I discovered that an arsonist had torched my tomato plants! All eight vines looked like pine trees after a forest fire.

The firebug was no human, though, but a disease: blight. In much of the East and Midwest, early and late tomato blight infestations are very hard to hold at bay. And when it rains every day for two months, as it did last summer in western North Carolina, it's impossible.

So it was with zeal that I drove out to North Carolina State University's Mountain Horticultural Crops Research Station in nearby Fletcher last September to visit the one person I knew would still have tomatoes on the vine: breeder Randy Gardner. Since 1976, Gardner has made battling blight his full-time job. He's been successful. Over the last several years, he has developed the early-blight-resistant Mountain series, including Mountain Pride, Supreme, Gold, Fresh and Belle.

Gardner did, indeed, have tomatoes. Row upon row of them -- red, green, yellow, orange, big, small, pear, plum -- his field beds a living gene library of Lycopersicon esculentum: the tomato. All had the stop-at-three-feet height of determinate varieties. Many were so laden with large fruits they looked like pint-size Christmas trees. He even had one I nicknamed the "anti-tomato." It was everything a tomato shouldn't be: a solid green, softball-hard globe that has the flavor of watermelon rind and, best of all, never ripens. Why keep such a gardener's nightmare alive? "It has a ripening inhibitor gene that's useful for crossbreeding," I was told.

Gardner was a pleasure to talk with. A straightforward, down-to-earth man, a farmer of genes, not crops, he talked in layman's language and exhibited a patient determination that reminded me of the land itself. We started off, of course, on the subject of b-l-i-g-h-t, but by the end of the day, we had also talked about tomato marketing, flavor, genetic engineering and more.

Early Blight (Alternaria solani) occurs wherever tomatoes are grown. It is most common in humid or semiarid climates where frequent dews provide sufficient moisture to permit disease development

Symptoms: Small, brownish black lesions form on older foliage first. The tissue surrounding the spots may become yellow, and by the time spots are about 1/2 inch in diameter, concentric rings may be discernable in the dark brown portion of the spot. When a spot is located on one of the primary leaf veins, the area beyond the spot soon dies and becomes brown. In the latter part of the season, the lesions become numerous and under favorable conditions, affected plants become defoliated, exposing fruit to sunscald. Stem lesions on seedlings are small, dark and slightly sunken. They enlarge to form circular or elongated lesions with pronounced concentric rings and light centers. If lesions continue to enlarge at the ground level, plants will become girdled and die. This phase of the disease is called collar rot.

Fruits are infected at either the flower or stem end, in either the green or ripe stage. Lesions attain considerable size, occasionally involving the entire fruit, and usually have concentric ringing. Diseased areas appear leathery and may be covered by a velvety mass of black spores.

Disease cycle: The soilborne fungus survives between crops on debris in the soil and on seed. In mild locations it survives from season to season on volunteer tomato plants and other related host plants, such as potato, eggplant, horsenettle and black nightshade. Primary infection occurs during periods of mild, 75° F to 85° F, rainy weather. Older leaves are affected first. High soil fertility reduces severity of early blight.

Late Blight (Phytophthora infestans) is the fungal disease that caused the Irish potato famine in the 1840's. It's also very serious on tomatoes when the weather is consistently cool and rainy.

Symptoms: The fungus attacks all aboveground parts of the tomato. At first, leaf lesions appear as water-soaked spots. They enlarge rapidly into pale green or brown spots that cover large areas of the leaf. In moist weather, the undersides of small lesions may be covered with a gray to white moldy growth. Infected foliage becomes brown, shrivels and dies. Petioles and stems are affected similarly. Fruit lesions appear as dark, greasy spots, which enlarge until the entire fruit is invaded. Soft rot often follows late blight infection and leads to fruit disintegration. Decaying vines may have a foul odor.

Disease cycle: The fungus survives between seasons in volunteer and abandoned potato and tomato plants in fields and gardens. Cool nights and warm days (temperatures below 85° F) are ideal for late blight development. The spores spread readily by wind to other plants and when weather is favorable, the infection moves so rapidly that affected plants appear as though damaged by frost.

Control: Eliminate all potato cull piles in the vicinity of tomato plantings, and destroy volunteer potato plants that grow from overwintered tubers.